Papillomavirus (PV) infections occur in a variety of animals, including humans, sheep, dogs, cats, rabbits, monkeys, snakes and cows. Papillomaviruses infect epithelial cells, generally inducing benign epithelial or fibroepithelial tumors at the site of infection. PV are species specific inflective agents; a human papillomavirus does not infect a nonhuman animal.
Papillomaviruses may be classified into distinct groups based on the host that they infect. Human papillomaviruses (HPV) are further classified into more than 70 types based on DNA sequence homology. PV types appear to be type-specific immunogens in that a neutralizing immunity to infection by one type of papillomavirus does not confer immunity against another type of papillomavirus.
In humans, different HPV types cause distinct diseases. HPV types 1, 2, 3, 4, 7, 10 and 26-29 cause benign warts in both norrnal and immunocompromised individuals. HPV types 5, 8, 9, 12, 14, 15, 17, 19-25, 36 and 46-50 cause flat lesions in immunocompromised individuals. HPV types 6, 11, 34, 39, 41-44 and 51-55 cause benign condylomata of the genital or respiratory mucosa. HPV types 16 and 18 cause epithelial dysplasia of the genital mucosa and are associated with the majority of in situ and invasive carcinomas of the cervix, vagina, vulva and anal canal.
Papillomaviruses are small (50-60 nm), nonenveloped, icosahedral DNA viruses that encode for up to eight early and two late genes. The open reading frames (ORFs) of the vines genomes are designated E1 to E7 and L1 and L2, where “E” denotes early and “L” denotes late. L1 and L2 code for virus capsid proteins. The early (E) genes are associated with functions such as viral replication and cellular transformation.
The L1 protein is the major capsid protein and has a molecular weight of 55-60 kDa. The L2 protein is a minor capsid protein which has a predicted molecular weight of 55-60 kDa and an apparent molecular weight of 75-100 kDa as determined by polyacrylamide gel electrophoresis. Immunological data suggest that most of the L2 protein is internal to the L1 protein within the viral capsomere. The L1 ORF is highly conserved among different papillomaviruses. The L2 proteins are less conserved among different papillomaviruses.
The L1 and L2 genes have been identified as good targets for immunoprophylaxis. Studies in the cottontail rabbit papillomavirus (CRPV) and bovine papillomavirus (BPV) systems have shown that immunizations with the L1 and L2 proteins expressed in bacteria or by using vaccinia vectors protected animals from viral infection. Expression of papillomavirus L1 genes in baculovirus expression systems or using vaccinia vectors resulted in the assembly of virus-like particles (VLP) which have been used to induce high-titered virus-neutralizing antibody responses that correlate with protection from viral challenge.
Following HPV type 16, HPV18 is the second most prevalent HPV type found in cervical carcinomas. HPV18 was detected in 5-20% of cervical cancer biopsies collected from various parts of the world (Ikenberg, H. 1990. Human papillomavirus DNA in invasive genital carcinomas. In Genital Papillomavirus Infections, C. Gross et aL, eds. p. 85-112). There appears to be a geographic dependence of infection with HPV18 since tumor biopsies from African and South American women harbor HPV18 more frequently than similar biopsies from European and North American women. The underlying reasons for these geographic differences are not known. The development of a vaccine against HPV18 infection becomes extremely relevant since HPV18 is also associated with more aggressively growing cancers and is rarely found in the milder precursor lesions, CIN I-II.